The present invention relates to an exposure control mechanism for a camera, and more particularly relates to an exposure control mechanism for a still video camera which obtains an image signal using a solid image sensor and records the image signal onto a floppy disk.
There are various kinds of exposure control mechanisms for cameras. In the case of silver halide photography, the most common exposure control mechanism is provided with a photometry sensor in order to conduct photometry.
In the case of a still video camera, negative feedback control is generally conducted in such a manner that: the output signal of an image pickup element is integrated; when the integrated value is small, the iris is opened; and when the integrated value is large, the iris is stopped down.
The inventors have already proposed an exposure control mechanism (which was disclosed in the official gazette in Japanese Patent Application No. 70409/1989) which is characterized in that: pixels are sorted according to their luminance; a histogram is made according to the number of pixels; and the most appropriate exposure conditions are determined according to the histogram.
The exposure control mechanism described above, which uses an exclusive photometry sensor, can not conduct exposure control of high accuracy because the sensitivity of the sensor and image sensor is varied.
In the case of the exposure control mechanism in which an exposure is controlled by a negative feedback control circuit using an image pick-up signal, the iris is gradually changed according to the integrated value of an image pick-up signal. Therefore, the following problem is caused: it takes a long time to adjust the iris, so that photographic opportunities may be missed.
The exposure control mechanism proposed by the inventors enables a photographer to catch all photographic opportunities, so that the mechanism is suitable for practical use.
The inventors have investigated the measures to improve the performance of a camera. As a result, the following problems have been clarified.
When investigating camera performance, the inventors ascertained that: when the pixels on an image screen are sorted according to the levels of luminance in order to make a luminance histogram, the histogram is made across the entire luminance range in which an exposure can be conducted, so that exposure must be conducted 3 to 4 times, and as a result it takes a long time to perform the photometry, and thus the response ability is lowered.
The reason why a plurality of exposures are needed is as follows: the range (the AE interlocking range) of luminance in the luminance histogram is usually spread from 7 to 18 Lv (Light value), so that a plurality of exposures of various exposure conditions are necessary in order to obtain the data of pixel distribution of the entire luminance range.
The following control method is conventionally used for an automatic exposure control (which will be called AE in this specification hereinafter) of a video camera: the iris and exposure time are controlled so that the averaged value of the integrated luminance signal level of the whole screen is proper.
Other than the method described above, various automatic exposure control methods have been developed such as: a method in which the data obtained from the center of the image screen is weighted; and a method in which an image screen is divided into a plurality of areas and the luminance signal level is integrated in each area.
Further, the conventional exposure control methods have the following problems: when the detected luminance belongs to an evaluated luminance range, consideration is not given to whether the pixel is on the high luminance region or in the low luminance region; and part of the obtained data, in the form of a histogram, is not utilized when the data is not in the range of the main photographic object.
As a result, the following problems occur, which will be explained as follows referring to the drawings.
The problem which is caused when all the data in a luminance range is equally treated will be explained as follows, with reference to the drawings.
For example, the case will be considered in which a histogram illustrated in FIG. 11 is obtained by preliminary exposure.
The width (the elemental luminance range) of the stripe shown in the histogram is set to 0.5 Lv in order to maintain the accuracy of the histogram. The width of the evaluation luminance range which is used as the reference to judge the luminance distribution, is set to 2 Lv. The numbers of pixels of four elemental luminance ranges located in the range of 2 Lv are added, and the sum is used as an evaluation value. The exposure conditions are determined according to the luminance range, the evaluated value of which is the highest. The reason why the width of the evaluation luminance range is set to 2 Lv, is as follows: an ordinary photographic object usually has a fairly wide luminance range, and therefore it is necessary to evaluate the luminance with the range of 2 Lv.
In the case of the histogram illustrated in FIG. 11, when evaluation is performed with the range of 2 Lv (in other words, the evaluation luminance range is 2 Lv), the evaluation value becomes highest in the range of 9 to 11 Lv. In this case, photographing is performed under the exposure condition (i.e. exposure condition A) in which the image pick-up signal of 11 Lv, which is the maximum luminance in this luminance range, can be 100%.
However, when exposure control is conducted in the manner described above, adequate exposure can not be obtained in the primary portion of the image, the luminance of which is in the range of 8 to 10 Lv, so that the obtained image becomes dark, which is not desirable. In this case, it is preferable to expose the image according to the luminance range of 8 to 10 Lv, in other words to control the exposure according to exposure condition of B. Even though the image of which the luminance range is 10 to 11 Lv becomes too bright, it is preferable to give an adequate exposure to the primary portion of the image, the luminance range of which is 9 to 10 Lv. In some cases, the most suitable exposure conditions can not be selected in the manner explained above.
The problem which occurs when the luminance information of other luminance ranges is not used, will be explained as follows.
For example, the case is considered in which the luminance histogram illustrated in FIG. 14 is obtained by a preliminary photographing. When an evaluation is performed with the range of 2 Lv, the maximum evaluation value can be obtained in the range of 8 to 10 Lv and photographing is conducted under the exposure condition of C. In the photographing described above, the signal level of the primary portion of the photographic object becomes adequate. However, although there are numerous picture elements in the luminance range of not less than 10 Lv, all of the portion of luminance which is more than 10 Lv becomes white, so that the area of the white portion becomes very large. Accordingly, the averaged luminance level of the image on a TV screen becomes abnormally high, which gives a bad impression. As explained above, even though the signal level of the primary photographic object is the most suitable, appropriate exposure can not be given to the image as a whole.
Further, there is a problem which will be described as follows: when exposure control is conducted according to information obtained by averaged overall photometry and by weighted averaged central photometry, normal exposure can not be obtained under the condition of excessively bright background lighting.
In order to cope with the situation described above, a camera having the following characteristics has come onto the market: when a photographer has judged that a normal exposure will not be obtained due to the condition of a photographic object, a corrected exposure can be conducted by a special function provided to the camera.
In this case exposure correction is conducted in such a manner that: the exposure values obtained by AE (an automatic exposure control mechanism) are corrected by adding, for example +0.5 EV, +1 EV, -0.5 EV and -1 EV.
In the case where exposure is controlled according to the information obtained by a multi-divided brightness metering such as an evaluation metering or a multi-pattern metering, the camera automatically detects photographing conditions such as bright background light, and a predetermined exposure correction is conducted according to a predetermined algorithm.
In the conventional exposure correction systems explained above, a photographer can easily comprehend the characteristics of the averaged overall metering system and the center-weighted metering system, so that when the photographer judges that he can not attain a proper exposure, for example when he takes a photograph of a landscape and a setting sun, focusing on the setting sun, he is aware that the subject will be taken in silhouette, and he can take the photograph under a roughly corrected condition.
However, in the methods described above, it is difficult to obtain an accurate correction value. Consequently, when the photographer desires an image of accurate exposure, he must take several photographs under various conditions, in other words, he must conduct bracketing, which takes time and costs much labor.
When exposure is controlled with evaluation metering or multi-pattern metering, a tentative correction can be automatically conducted according to the obtained information. However, due to the bias and accuracy of information, and due to the composition of the control algorithm, the range of correction is limited. Therefore, in some cases a wrong exposure correction is conducted, which leads to a failure of photographing.
When the photographer tries to avoid the problems described above, it is difficult for him to take proper measures against the problems because the control algorithm is sophisticated. As described above, the reliability of automatic exposure correction is limited and it is difficult for a photographer to correct the exposure conditions according to his skill. Accordingly, in the case where an accurate exposure is necessary, there are problems which can not be solved by conventional technology.